This invention relates generally to gas turbine engines, and more specifically to methods and apparatus for operating gas turbine engines.
Gas turbine engines generally include, in serial flow arrangement, a high-pressure compressor for compressing air flowing through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high temperature gas stream, and a high-pressure turbine. The high-pressure compressor, combustor and high-pressure turbine are sometimes collectively referred to as the core engine. Such gas turbine engines also may include a low-pressure compressor, or booster, for supplying compressed air to the high-pressure compressor.
To improve performance, the gas turbine engine is optimized for full load operation so as to reduce fuel consumption while still providing the desired power. Similarly, the combustion system is optimized for full load operation to reduce emissions during normal operation. For modern premixed combustion designs, this optimization drives a maximum of the available airflow to the premixing device to keep combustion temperatures low and minimize the formation of oxides of nitrogen, which are a strong function of temperature. It is well established that the rate of formation of oxides of nitrogen is a function of the peak reaction temperatures, and they themselves are driven by the fuel to air ratio in the combustor. Fuel to air ratios on either side of the stoichiometric ratio result in lower peak temperatures, with operation on the “lean” side (i.e. more air than fuel) as the preferred embodiment for dry low NOx designs.
However, during some operational conditions, it is desirable to operate the gas turbine engine at reduced power levels. To reduce the power output on the gas turbine, the fuel, the air, or the rotational speed can be reduced. Most large heavy duty gas turbines operate at a fixed mechanical speed, hence to reduce power, they rely on decreasing fuel flow and air flow. If only the airflow is reduced, the power output is decreased, and operating temperatures in the combustor may exceed their intended design point. If only fuel flow is reduced, the combustor may approach lean stability limitations. In general, power is reduced by decreasing together both air flow and fuel flow, while at the same time mitigating lean stability issues by the use of pilots or fuel staging (such as in a multi-nozzle device). Nevertheless, regardless of the approach, a condition is reached while trying to reduce power, where the combustor operating temperatures may be too low to complete combustion of the fuel, which may result in an increase in pollutants such as carbon monoxide, for example.